The present invention is generally directed to methods, systems, and apparatus for laser calibration and eye tracker camera alignment. In particular, the present invention relates to methods and systems for measuring laser energy, shape, and dimensions of a laser beam from a laser beam delivery system, particularly opthalmological surgery systems, and aligning eye tracking cameras used in conjunction with such laser systems that measure a position of the eye during laser eye surgery.
Laser-based systems are now used in opthalmological surgery on corneal tissues to correct vision defects. These systems use lasers to achieve a desired change in corneal shape, with the laser removing thin layers of corneal tissue using a technique generally described as ablative photodecomposition to alter the cornea""s refractive power. Laser eye surgery techniques are useful in procedures such as photorefractive keratotomy (PRK), phototherapeutic keratectomy (PTK), laser in situ keratomileusis (LASIK), and the like.
In such laser-based systems and methods, the irradiated flux density and exposure time of the cornea to the laser radiation are controlled so as to provide a surface sculpting of the cornea to achieve a desired ultimate surface change in the cornea. To that end, ablation algorithms have been developed that determine the approximate energy density that must be applied to remove a certain depth of tissue from the cornea. At ultraviolet wavelengths, for example, a cumulative energy density of about 1 joule/cm2 will typically ablate corneal tissue to a depth of about one micron when applied in a series of pulses of about 40 to 400 millijoules/cm2. Accordingly, the ablation algorithms are tailored for each procedure depending on the amount and the shape of corneal tissue which will be removed to correct a particular individual""s refractive error.
In order to properly use these laser ablation algorithms, the laser beam delivery system typically should be calibrated. Calibration of the laser system helps ensure removal of the intended shape and quantity of the corneal tissue so as to provide the desired shape and refractive power modification to the patient""s cornea. For example, deviation from a desired laser beam shape or size, such as the laser beam exhibiting a non-symmetrical shape or an increased or decreased laser beam diameter, may result in tissue ablation at an undesired location on the patient""s cornea which in turn leads to less than ideal corneal sculpting results. As such, it is beneficial to know the shape and size profiles of the laser beam so as to accurately sculpt the patient""s cornea through laser ablation. In addition, it is usually desirable to test for acceptable levels of system performance. For example, such tests can help ensure that laser energy measurements are accurate. Ablations of plastic test materials are often performed prior to laser surgery to calibrate the laser energy and ablation shape of the laser beam delivery system. Although such laser ablation calibration techniques are fairly effective, in some instances, alternative methods for laser energy and beam shape calibration may be advantageous.
A variety of integrated structures have been proposed for both scanning of a laser beam across the corneal tissue and tracking of eye movements. Tracking of the eye during laser eye surgery has been proposed to avoid uncomfortable structures which attempt to achieve total immobilization of the eye. Tracking further compensates for eye movement during a treatment procedure so that the intended portion of the eye may be accurately ablated. An exemplary two camera off-axis eye tracker for laser eye surgery is described in U.S. Pat. No. 6,322,216 B1, assigned to the assignee of the present application, the full disclosure of which is incorporated herein by reference. In this system, first and second cameras or image capture devices are oriented toward the eye. An energy delivery system laterally deflects an energy stream toward the corneal tissue along a first and second axis in response to movement of the eye sensed by the first and second image capture devices. Alignment of such image capture devices may be facilitated by a jig plate.
In light of the above, it would be desirable to provide improved methods, systems, and apparatus for calibrating laser energy, laser beam shape, and/or laser beam dimensions from a laser eye surgery system. It would be particularly desirable if such improvements enhanced calibration accuracy without significantly increasing the overall system cost and complexity. It would be further desirable if such methods, systems, and apparatus further allow for eye tracker camera alignment so that laser calibration and camera alignment may be conveniently and effectively carried out utilizing a single, reusable apparatus. At least some of these objectives will be met by the methods, systems, and apparatus of the present invention described hereinafter.
The present invention provides methods, systems, and apparatus for calibrating a laser ablation system, such as an excimer laser system for selectively ablating a cornea of a patient""s eye. The invention also facilitates alignment of eye tracking cameras (which are often used in conjunction with such laser systems) that measure a position of the eye during laser eye surgery. In particular, the present invention provides methods and systems which measure laser energy, laser beam shape, and/or laser beam dimensions with enhanced calibration accuracy without significantly increasing the overall system cost and complexity. Moreover, the present invention allows for laser calibrations and eye tracker camera alignment to be effectively and conveniently carried out.
In a first aspect of the present invention, a method for calibrating laser energy from a laser eye surgery system comprises transmitting or reflecting a laser beam suitable for ablation of corneal tissue from a surface, such as a galvanometric mirror having a reflecting surface or a beam splitter, and scanning the laser beam across a calibration fixture having a feature, such as an opening, reference-edge, or artificial pupil. Sample laser energy is separated from the beam at the surface during the scanning and measured. Laser energy transmitted past or directed at the feature during the scanning is measured. A calibration of the laser system is then determined by comparing the energy measurements. Energy measurements during the scanning are typically made by energy sensors, such as a photodetectors, light detectors, energy meters, and like detectors that are positioned near, adjacent to, or behind the surface or calibration fixture.
Calibration of the laser system is determined by comparing a ratio of the measured sample laser energy and the measured laser energy directed at the feature to a predetermined tolerance. If the ratio is within the predetermined tolerance, calibration of the laser system further comprises independently comparing the measured sample laser energy to a first threshold range and the measured laser energy directed at the feature to a second threshold range. Calibration of the laser system is complete if the measured sample laser energy is within the first threshold range and the measured laser energy directed at the feature is within the second threshold range. In a passing calibration, the calibration fixture may then be removed from a treatment plane and the laser beam directed towards a patient""s cornea for ablating the cornea with the calibrated system. However, if the measured sample laser energy is outside the first threshold range or the measured laser energy directed at the feature is outside the second threshold range, a fault is indicated in the laser system, such as flawed delivery system optics or a flawed laser.
The laser beam may be transmitted from several different positions on the surface. In the case where the ratio is outside the predetermined tolerance, each ratio of the measured sample laser energy and the measured laser energy directed at the feature is analyzed for each laser beam position on the surface to determine if the ratio is position independent. If the ratio is not position independent, a fault is indicated either in the surface or an energy sensor that measures sample energy or laser energy directed at the feature. If the ratio is position independent, a fault is indicated either in the energy sensor that measures sample energy or laser energy directed at the feature or the laser beam delivery system. Calibration of the laser system may further indicate if an energy sensor measures the sample energy or laser energy directed at the feature at an accuracy within a predetermined threshold.
As described above, the surface preferably comprises a mirror having a reflecting surface, wherein a photodetector measures sample energy, such as laser energy leakage through the mirror. The feature comprises an opening in the calibration fixture which is positioned adjacent a treatment plane, wherein a photodetector measures laser light energy passing through the opening. A variation in each photodetector due to spatial non-uniformity is further measured prior to laser beam scanning across the calibration fixture to separate this effect from the laser energy calibration calculations described above. Moreover, a large number of measurements are made so that contributions due to detector noise are relatively insignificant as compared to an average of laser energy measurements. The tolerance and threshold values will depend on the level of calibration accuracy desired. For example, the predetermined ratio tolerance provides preferably 8% or less inaccuracy, more preferably 4% or less inaccuracy, most preferably 2% or less inaccuracy while the threshold values may provide 1% or less inaccuracy. The laser beam will typically be oriented perpendicular to the calibration fixture. The present methods advantageously allow for enhanced laser energy calibration as energy measurements from two photodetectors are used to determine an accurate calibration of the laser system. Moreover, energy measurements from two photodetectors allow for fault detection within the laser system to be narrowed to a specific source(s), which in turn facilitates fast and accurate adjustment of the laser system.
The calibration feature may further comprise a first reference-edge, such as a knife-edge, so as to determine a characteristic of the laser beam by measuring laser energy passing the first reference-edge during scanning with a photodetector. Multiple measurements are generated as the laser beam is fully incident on the first reference-edge (i.e. the laser beam is fully blocked from reaching the photodetector by the reference-edge) to the laser beam being fully incident on the photodetector (i.e. the laser beam is not blocked by the reference-edge). The calibration feature will preferably comprise a second reference-edge oriented at an angle relative to the first reference-edge. A characteristic of the laser beam may be determined by measuring laser energy passing the second reference-edge during scanning.
An intensity profile of the laser beam may be determined from the measured laser energy passing the first or second reference-edges during scanning. The scanning laser beam provides an integration of the laser beam intensity profile. Dimensions of the laser beam may then be determined from the laser beam intensity profile. For example, dimensions of the laser beam may be determined by finding positions of the laser beam along the two orthogonal reference-edges where the measured laser energy passing the reference-edge during the scanning reaches a certain percent of a maximum signal. In some instances, the intensity profile of the laser beam may be verified so that it is within a predetermined acceptable range from the compared energies. A shape of the laser beam may further be determined by measuring a rate of change of the measured laser energy passing the reference-edge during scanning. Laser beam shape and dimension measurements provide information on beam quality, such as ellipticity, eccentricities, and asymmetries in the laser beam, which in turn facilitates accurate sculpting of the cornea.
The calibration feature may further be imaged with an image capture device of an eye tracker system so as to align the image capture device with the laser system. In such instances, the calibration feature may comprise four dark circles that preferably emulate eye pupils disposed at four corners of a square. Alternatively or additionally, the imaged feature may comprise an opening or a reference-edge.
In another aspect of the present invention, methods for characterizing a scanning corneal ablation laser beam are provided. One method comprises scanning a laser beam across a calibration fixture having a reference-edge, measuring the laser beam energy passing the reference-edge while scanning the laser beam, and deriving a characteristic of the laser beam from the measured laser beam energy. The calibration fixture may be removed following measurement and the a patient""s eye treated by ablating the patient""s cornea with the measured laser beam.
In yet another aspect of the present invention, systems for calibrating laser energy from a laser beam delivery system are provided. Such systems may comprise a scanning laser beam delivery system, preferably a laser eye surgery system, a surface that directs laser energy from the laser beam delivery system toward a treatment plane, the surface separating a sample laser energy from the beam, a first photodetector positioned in a first optical path of the sample laser energy from the surface, a calibration fixture positioned adjacent the treatment plane, and a second photodetector positioned in a second optical path of the laser beam from the feature of the calibration fixture. The first photodetector emits a first output signal in response to the sample laser energy, for example, the amount of laser energy leakage through the surface or mirror. The second photodetector emits a second output signal in response to the laser beam incident thereon. A processor is also included in the system to determine a calibration of the laser system or a characteristic of the laser beam in response to the first and second output signals.
The calibration feature may comprise an opening in the calibration fixture that is sufficiently large so that a whole of the laser beam can pass through it. The calibration feature may further comprise a reference-edge or two reference-edges so that the laser beam is directed from the surface across each reference-edge so as to determine a characteristic of the laser beam (e.g., shape, dimension). In an exemplary embodiment, the calibration feature has a cross-like pattern comprising twelve reference-edges so as to allow for multiple measurements which in turn enhances laser beam dimension and shape measurements. The system may further comprise an image capture device orientated toward the treatment plane and an image processor coupled to the image capture device. The image processor determines a position of the calibration fixture for verification of alignment between the image capture device and the laser delivery system. In such instances, the imaged calibration feature preferably comprises four dark circles disposed at four corners of a square pattern.
In a still further aspect of the present invention, a calibration and alignment fixture for a scanning laser beam delivery system having at least one image capture device may comprise a structure positionable in a treatment plane. The structure has a feature directing laser energy incident thereon to a calibration energy sensor, at least one reference-edge to determine a characteristic of the laser beam (e.g., shape, dimension), and an artificial pupil to determine alignment of the at least one image capture device with the laser system. The calibration feature comprises an opening that is sufficiently large so that a whole of the laser beam can pass through it. Preferably, the calibration fixture has two reference-edges, the second reference-edge oriented at an angle relative to the first reference-edge, more preferably the fixture has a cross-like pattern comprising twelve reference-edges. The artificial pupil may comprise a dark circle, preferably four dark circles disposed at four corners of a square, or an opening or hole in the fixture. The cross-like pattern of twelve reference-edges may also be used to align the image capture device with the laser system. Conveniently, laser calibration and laser beam characteristics may be determined and eye tracking cameras aligned effectively via a single, reusable fixture.
In another aspect of the present invention, a method for calibrating a laser eye surgery system having eye tracking cameras is provided. A position of a laser beam is measured, a position of a calibration feature is measured, and the measured position of the laser beam is compared to the measured position of the calibration feature. If the measured positions are within a predetermined tolerance, the eye may be treated via corneal ablation.
A further understanding of the nature and advantages of the present invention will become apparent by reference to the remaining portions of the specification and drawings.